Yagi-Uda antennas were originally described in the English language in an article written by H. Yagi (See H. Yagi, “Beam Transmission of the Ultra Short Waves,” Proc. IRE. Vol. 16, pp. 715-741, June 1928). These directional dipole antennas, which are commonly referred to as Yagi antennas, have been used for many years and in many applications. For example, the Yagi antenna has been used for reception of television signals, point-to-point communications and other electronics applications.
The basic Yagi antenna typically includes a driven element, usually a half-wave dipole, which is driven from a source of electromagnetic energy or drives a sink of electromagnetic energy. The antenna also typically includes non-driven or parasitic elements that are arrayed with the driven element. These non-driven or parasitic elements generally comprise a reflector element on one side of the driven element and at least one director element on the other side of the driven element (i.e., the driven element is interposed between the reflector element and the director element). The driven element, reflector element and director element are usually positioned in a spaced relationship along an antenna axis with the director element or elements extending in a transmission or reception direction from the driven element. The length of the driven, reflector and director elements and the separations between these antenna elements specify the maximum Effective Isotropic Radiated Power (EIRP) of the antenna system (i.e., directive gain) in the antenna system's bore site direction.
Current trends in antenna designs reflect the desirability of low profile, directional antenna configurations that can conform to any number of shapes for a mobile or portable unit while providing highly directional antenna patterns, such as those achievable with the Yagi antenna. In addition, current trends in antenna designs reflect the desirability of the antenna to maintain structural shape and integrity after application of an external force, such as a surface impact. Such antenna designs are particularly desirable in portable or hand-held devices such as cellular telephones, satellite telephones and contactless interrogators of Automatic Identification (Auto ID) systems, such as Radio Frequency Identification (RFID) interrogators of RFID systems.
RFID systems may include different tags with various frequency range response. For example, some RFID tags may be categorized as near-field tags or far-field tags based on a corresponding frequency designation for such tags. Additionally, RFID tags may operate in different frequencies. One design consideration for RFID systems is that an RFID antenna used for reading such tags generally has satisfactory reading operation when the antenna size is physically large. Another design consideration for RFID systems is that with hand-held and hands-free RFID readers incorporating RFID antennas, a minimized antenna size is generally desired for portability. An additional concern is that a closer proximity of the RFID antenna to a part of a human body tends to degrade performance of the antenna.
Accordingly, it is desirable to provide a multi-frequency, low profile, directional antenna having highly directional antenna patterns. In addition, it is desirable to provide a multi-frequency, directional antenna having near-field and far-field elements while maintaining a relatively compact size. Furthermore, it is desirable to provide such an antenna for portable or hand-held devices that has low detuning sensitivity from user proximity. Moreover, desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.